Compared to other transparent infrared fiber materials, ZBLAN fluoride glasses promise to be best suited for laser power
delivery in the 3μm wavelength region due to their high transmission and excellent mechanical flexibility. These claims
were demonstrated in a series of power handling tests of both straight and coiled fibers using an Er,Cr:YSGG laser
emitting a train of pulses of 150 μs duration at a repetition frequency of 20 Hz producing 7.5 W average power. Large
core fibers (450/510μm 0,2NA) are characterized by an attenuation of 0.02dB/m at 3μm and stay within 0.5°C from
ambient temperature when carrying full laser power. A 2-m fiber length prepared with bare cleaves has been tested for
over 23 hours, cumulating 1,140,000 shots of 1530 J/cm2 fluence while maintaining 90% transmission without any
measurable degradation. Coiling the fiber to 11 cm radius did not have an impact on power handling reliability. These
results show the potential of these highly transparent fibers in surgical laser delivery applications.
In order to test power-handling at 1kW, a special splitter component had to be developed to make use of available
sources. A tapered fused-bundle (TFB) 1X7 splitter using a 1.00mm core diameter 0.22NA input fiber coupled to seven
400 micron core 0.22 NA output fibers was tested up to 860W at 976nm. Surface temperature rise was measured to be
less than 15°C with active heat sinking. The above results suggest that understanding the mechanisms of optical loss for
forward and backward propagating light in a TFB and the heat load that these losses generate is the key to producing
multi kW components, and demonstrates that reliable kW-level all fiber devices are possible.
Light absorption in structural adhesives constitutes the main source of heat in tapered fused bundle (TFB) devices.
Efficient heat dissipation solutions were developed by studying these thermal loads. The relative merits of transparent
vs. opaque package designs were established experimentally. In the former, light escapes without being absorbed by the
package walls, whereas in the latter, the spurious optical signal is directly absorbed and dissipated. The fact that heat is
generated directly in the adhesive largely favors the opaque package, which offers more efficient heat extraction. By
using a thermally conductive package, a temperature rise of 1.1°C per Watt of dissipated power was measured. These
numbers demonstrate that passive heat sinking at 20°C is sufficient to allow reliable operation up to 45Watts of
dissipated power (1kW with 0.2dB optical loss) without compromising long-term reliability.
In this paper we review the damage mechanisms that need to be considered when building high power fibre lasers. More specifically we look at thermal issues, optically induced coating damage, bulk and surface damage thresholds of the host glass. We also discuss the reliability of tapered fibre bundles and Bragg gratings at these power densities.
Fiber lasers have shown extraordinary progress in power level, reaching the kilowatt range. These results were achieved with large mode area fibers pumped with high power laser diodes coupled with bulk-optics. To enable the commercial development of these high power fiber lasers, we have demonstrated several All-Fiber components, which replace the bulk-optic interface in the present laser configurations. These components include multimode fused fiber bundle combiners with or without signal fiber feed-through, Bragg gratings and mode field adaptors. The multimode fibers are used to couple several fiber pigtailed pump diodes to a double-clad fiber. Such combiners may contain a signal fiber to provide an input or output for the core modes of the double-clad fiber. Mode field adaptors perform fundamental mode matching between different core fibers. Bragg gratings are used as reflectors for the laser cavity. These components exhibit low-loss and high power handling of 200 Watts has been demonstrated. They enable the design of true high power single-mode All-Fiber lasers that will be small, rugged and reliable.
Stress corrosion factors were obtained for the waist region of a fused fiber component using the dynamic fatigue method at ambient temperature and humidity. The n-value obtained is 23.3, which is comparable to that of pristine fiber. The ultimate tensile strength of the waist is much higher than that of the mechanically stripped fiber used to manufacture the components, indicating that flaws and defects are actually repaired during the fusion process. Component lifetime and reliability are estimated.
A three-branch integrated-optic Mach-Zehnder interferometer is proposed for the simultaneous measurement of pressures and temperature. A broadband source coupled with a spectrometer is used to perform phase measurements independently from transmission loss fluctuations. Fast Fourier transform techniques applied to the transmitted spectrum allow demultiplexing of the signal from each pair of branches. Various correlation criteria are applied to establish pressure and temperature values from fractional phase measurements.
Continuous wave (cw) and Q-switched fiber lasers were implemented using highly doped erbium fiber pumped with a 980-nm pigtailed laser diode. Thresholds and slope efficiencies were characterized for various reflector combinations to achieve optimal cw operation. An output coupling of 90% was found to produce maximum output power of 30 mW at 1560 nm with 78 mW of launched pump power. Pulsed operation was achieved with low loss on the first diffracted order of a bulk acousto-optic Q-switch. Output coupling from the fiber laser was obtained through an intrinsic fiber grating. Available pump power levels of 78 mW produced 20-ns pulses of 250 W peak power and a repetition rate of up to 3.5 kHz. The effects of repetition rate, cavity losses and pump power on the pulsewidth, peak power and lasing spectrum were characterized.
A Q-switched fiber laser is developed for eyesafe ranging applications. Using a highly doped erbium fiber pumped with a 30-mW laser diode at 980 nm, pulses of 290-W peak power and 20-ns duration are obtained. The laser consists of all-fiber elements, with the exception of a high-efficiency bulk acousto-optic Bragg cell aligned to the laser on the diffracted beam to provide Q-switching with high hold-off. Three emission bands of erbium are made to lase in continuous wave (cw) and Q-switched operation. It is established that optimal cw lasing is achieved at 1560 nm, and maximum intensity Q-switch pulses are produced at 1530 nm. The laser was packaged and tested in the field, demonstrating the readiness of this technology for ranging applications.